The invention relates to a method of determining the optical quality of a fluoride single crystal, and the making of  less than 200 nm lithography laser optical elements.
There currently exists a strong demand for optical lithography exposure tools which function at 193 and 157 nm excimer laser wavelengths.
In this regard, optical fluoride crystals are used as optical elements in illumination systems, projections systems and excimer lasers, due to their transmission and durability which make them optical materials which are ideal for applications at wavelengths of less than 200 nm.
This region of the electromagnetic spectrum is of great interest, in particular for high resolution microlithography systems.
A microlithography system contains sub-systems which are mainly the illumination system and the projection system.
The projection system is for focalising the picture and therefore the optical elements which include it must be of a superior optical quality.
In this respect, the optical homogeneity of the crystals must be less than 2 ppm, and preferably less than 1 ppm, for applications in projection systems.
As for the illumination system, this is for forming the laser beam and the requirements upon the crystalline quality of the optical elements which include it are also very high, although lower than for the projection system.
Up to now, the quality of the single crystals constituting such optical elements was optically checked by measuring the birefringence, the optical homogeneity, and by a general visual inspection of the single crystals. However, these techniques only offer an optical macroscopic view of the quality of the crystal. The commercial use and adoption of 193 nm and below 200 nm vacuum ultraviolet wavelengths such as 157 nm has been hindered by the transmission nature of such deep ultraviolet wavelengths in the 157 nm region through optical materials. Such slow progression by the semiconductor industry of the use of VUV light below 200 nm such as the 157 nm region light has been also due to the lack of economically manufacturable blanks from optically transmissive materials and difficulties in manufacturing blanks which can be identified as high quality and qualified for their intended microlithography optical element and laser use. For the benefit of below 200 nm deep ultraviolet photolithography in the VUV 157 nm region such as the emission spectrum of the fluorine excimer laser to be utilized in the manufacturing of integrated circuits there is a need for below 200 nm wavelength transmitting optical fluoride crystals that have beneficial optical and highly qualified properties including good transmission below 200 nm and at 193 nm and 157 nm and that can be manufactured reliably and economically. The present invention overcomes problems in the prior art and provides a means for economically providing high quality below 200 nm wavelength transmitting optical fluoride crystals, element blanks, and elements that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths. The invention provides for determination and selection of high quality calcium fluoride optical fluoride crystal lithography and excimer laser elements with low mosaicity levels.